1. Field of the Invention
The present invention generally relates to charged particle beam (e-beam) deflection systems and, more particularly, to a deflection system employing two sets of windings on the same core intended to provide two independent channels of dynamic electron beam positioning with different time responses.
2. Description of the Prior Art
In applications of charged particle beam and specifically of electron beam technology, for example, the lithography of integrated circuit patterns, speed of deflection is of critical importance to obtaining practical throughputs for an electron beam deflection system design. In a typical system design, a number of stages of beam deflection are used to better amortize the settling times of deflection so that an effectively shorter settling time is realized and therefore a higher throughput is achieved.
In almost all e-beam lithography systems designed in the last twenty years or so, at least two stages of deflection have been incorporated. One stage of magnetic deflection and one stage of electric deflection. The magnetic deflection is used to deflect the beam at relatively large distances with high precision; however, long settling times are required to achieve this precision. The electric deflection is then used to deflect the beam within a relatively small area but at substantially higher speeds. Thus, the beam is deflected within the total field of the deflection system, to each of a relatively small number of subfields at relatively slow speeds by the first stage of deflection. The beam is then deflected to a large number of beam or pattern locations within each of the subfields by the second stage of deflection at very fast speeds. As a result, the two stage deflector is much faster for the same precision and deflection field size than is the single stage deflector. The reason for this is that precision and speed act counter to each other. A high precision deflector requires more time to settle than does one of less precision for the same maximum deflector range. Also, magnetic deflectors are practically much easier to deflect over large areas than are electric deflectors.
U.S. Pat. No. 5,136,167 describes an e-beam lithography system in which an additional stage of magnetic deflection is incorporated so that larger deflection fields can be achieved at even faster speeds. This is achieved by elaborate means to retain telecentric deflection for all three stages of deflection. This system also incorporates telecentric and low aberration deflection over large areas.
This presents a problem if the two stages of magnetic deflection are used where the major and minor deflectors are required to be at the same position along the length of the beam. This is the case when there is limited space in which to position the two yokes or as in the Variable Axis Immersion Lens (VAIL) where the two deflection yokes (referred to as correction yokes in VAIL) must both generate identical magnetic fields, to cancel the first derivative of the axial lens field. This implies that the two fields are directly coupled magnetically and therefore inductively. This is where the problem arises. If the two yokes are inductively coupled, the speed of deflection cannot exceed the slowest yoke speed. This comes about because if the faster yoke is excited and it is inductively coupled to the slower yoke and the excitation of the faster yoke will induce an excitation in the slower yoke which will then settle to the required precision in the same time as would be the case if the slower yoke were initially excited. In order to make two stages of deflection, two identical magnetic fields must be generated by two different deflection yokes located in the same location and yet remain inductively decoupled.
In addition, for large field deflection system design, high sensitivity is required to keep the electronic driver circuitry within practical limits. The most efficient way to achieve this, particularly with toroidally wound yokes, is by the use of ferromagnetic cores. Ferrite is the most common core material because it obtains the high magnetic permeability required for a good core material but does not support the generation of eddy currents which can slow the settling time and therefore the deflection speed. However, if the core material is immersed in a lens field or other deflection fields, it can alter this field in a deleterious way. Most deflection system designs today are designed in conjunction with a lens to optimize aberrations and distortions of the deflection of an electron beam. Thus, the placement of a given deflection yoke is dictated by this optimization. In the case of the variable axis immersion lens (VAIL) type deflection system described in U.S. Pat. No. 4,544,846, the yoke must be fully immersed in the lens field at the location of the peak of the first derivative with respect to 2. If a yoke with a typical ferrite core were to be placed inside a magnetic lens, the core material would short the field inside the lens and cause large distortion and aberrations.